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The inevitable close association between water loss and entrance of carbon dioxide through stomates, together with cell-morphological and physiological traits that affect these exchanges, prevent plants capable of high rates of photosynthesis per unit leaf surface area when the soil is moist from being able to extract moisture from dry soils, and vice versa. Hypothetical photosynthesis curves for leaves of mesophytic and xerophytic plants are inferred from these associations and shown to be similar to those obtained from desert shrubs. Because they are cheaper to build and maintain per unit surface area, mesophytic leaves are amortized quickly and yield profits at faster rates than xerophytic leaves, provided soil water potential is low. Which type of leaf is the most advantageous to a plant depends on the length of time during the year that soil moisture is high and on the extent of periods of high soil water potential during otherwise wet periods that necessitate either closing stomates or dropping leaves. Costs of root construction and maintenance are poorly known, but it is probably more expensive to maintain roots under conditions of high soil and plant water potential. The optimal root/shoot ratio of a plant with mesophytic leaves should be close to that providing sufficient moisture for maximum photosynthesis during the wet season, while plants with xerophytic leaves should have an R/S ratio well below the optimal for maximizing photosynthesis. The model provides a reasonable explanation for the large numbers of desert plants with deciduous, mesophytic leaves and for the great variations reported in R/S ratios in desert plants. It also can explain why dry desert flats are dominated by evergreen sclerophyllous plants, while closer to the washes there is a zone of drought-deciduous perennials, replaced again by evergreens in the washes.